Bandgap-engineered iron-coordinated quercetin for enhanced visible-light photodynamic inactivation and strawberry preservation

Natural photosensitizers are promising for postharvest fruit preservation, but their antibacterial efficacy is often limited by weak visible-light responsiveness. Based on the natural photosensitizer quercetin, this study prepared an Fe-quercetin coordination complex (QF), which was subsequently incorporated into an edible coating with a pectin matrix for the preservation of strawberries. DRS-Tauc analysis showed that iron coordination reduced the bandgap of quercetin from 2.64 eV to 1.86 eV, and EPR analysis confirmed the generation of 1 O 2 , •O 2 − , •OH, and HO 2 • under illumination. Under visible-light irradiation, QF completely inactivated Staphylococcus aureus at 0.2 mg/mL within 10 min and Escherichia coli at 0.3 mg/mL within 5 min, whereas free quercetin was markedly less effective under the same conditions. In strawberries stored at 25 °C for five days, the QF-P coating delayed spoilage, reducing weight loss from approximately 16.63% to approximately 7.95%, while maintaining a firmness of approximately 242 g. QF-P coating also better preserved the pH, total soluble solids and color of strawberries during storage. These results show that iron coordination is an effective way to improve the visible-light-driven antibacterial activity of quercetin and support the use of QF-based edible coatings for preserving perishable fruits under mild conditions.